This invention relates to a tri-element carbon based heat shield, and more particularly, to a tri-element carbon based heat shield that beneficially employs three forms of carbon components according to the density, thermal diffusivity and structural strength of each.
One class of heat shields is used to protect components from severe and harsh environments, such as may be encountered by a re-entry type vehicle. Although carbon-carbon re-entry vehicle heat shields have advantages over carbon-phenolic type heat shields, the higher thermal conductivity of carbon-carbon heat shields and lack of an organic sacrificial thermochemical degrading component generally requires a backface, or inwardly disposed, thermal insulation layer which may result in a thermal protection shield that is too heavy, or, if a sufficient thickness of low density insulation is used to compensate for a lesser amount of high density material, an undesirable shift in the center of mass of the re-entry vehicle that disturbs the aerodynamic stability of the re-entry body may be experienced. Carbon-carbon based heat shields are typically used to reduce nuclear radiation absorption in the heat shield and for their desirable particle impact resistance, ablation characteristics and improved structural strength at the expected elevated operating temperatures.
U.S. Pat. No. 4,472,476--Vetri et al discloses a multilayer coating system for protection of carbon-carbon composites. In one embodiment, a layer of pyrolytic graphite is applied to the surface of the carbon-carbon material for obtaining a uniform surface condition for subsequent coating processes. It is the coating that is operationally exposed to the high-temperature environment and not the carbon-carbon material as contemplated by the present invention. U.S. Pat. No. 4,487,799--Galasso et al, having the same named inventors as U.S. Pat. No. 4,472,476, discloses a method for applying a layer of pyrolytic graphite to a carbon-carbon material for preparing the carbon-carbon material to receive another coating.
U.S. Pat. No. 4,442,165--Gebhardt et al discloses and claims a carbon-carbon syntactic foam material which is covered and penetrated by pyrolytic carbon. Pyrolytic graphite is then deposited on a surface of the pyrolytic carbon covered material for improving the thermal insulation of the composite and increasing the strength of the foam material. The pyrolytic graphite layer does not contact the carbon-carbon material and further, no layer of insulation for covering the pyrolytic graphite layer is disclosed.
U.S. Pat. Nos. 3,980,105--Myskowski and 3,853,583--Olcott disclose pyrolytic graphite deposited on a carbonaceous substrate and a carbon substrate, respectively.
Prior carbon-based heat shield structures have included: a combination having an outer shell of dense polycrystalline graphite (e.g. ATJ grade, about 1.6-1.75 gm/cc available from Union Carbide) or a medium density carbon-carbon composite with pyrolytic graphite deposited inside; a combination having an outer shell of polycrystalline graphite (e.g. ATJ grade) or a carbon-carbon composite that includes a low density (about 0.12-0.20 gm/cc) carbon felt or foam inner lining; or a heat shield of moderate density (about 1.4-1.6 gm/cc) carbon-carbon composite (such as Pyro-Carb 406 available from Hitco Corp, Gardena, Calif.), used with no backface insulation.
When used with a dense polycrystalline graphite outer primary shield, a pyrolytic graphite liner backface layer has tended to delaminate, either from the deposition substrate (adhesively) or within itself (cohesively), especially as greater thicknesses of liner are reached. Moreover, the relatively hard, stiff and brittle bi-element heat shield is difficult to bond to a rigid substructure, such as a missile nosecone, because of the low mechanical compliance of the combined heat shield/structure .
A polycrystalline graphite or carbon-carbon composite primary shield used in combination with a carbon or foam backface insulator suffers primarily in weight and thickness comparisons to primary heat shields composed of refractory-reinforced/resin matrix ablative heat shields, such as carbon phenolic. Disadvantages of a primary moderate density heat shield used with no backface insulation component include having higher thermal diffusivity at high temperature than a dense polycrystalline graphite, and also higher thermal diffusivity at low temperature than low density felt or foam insulations. This higher thermal diffusivity requires such a thermal protection system to be heavier and thicker than the multiple element heat shields for obtaining the same relative degree of protection.
Accordingly, it is an object of the present invention to improve the thermal protection efficiency of carbon based heat shied systems while minimizing weight and thickness requirements.
Another object is to improve the thermal performance of the heat shield while maintaining acceptable performance against nuclear radiation.
Yet another object of the present invention is to provide a carbon based heat shield offering adequate thermal protection without exceeding aerodynamic stability requirements.